The present invention is related to an improved method for forming a capacitor and improved capacitors formed thereby. More specifically, the present invention is related to the use of weak acids, preferably in combination with amines and more preferably monoamines, as a coating additive particularly for intrinsically conductive polymers. Superior coverage of a surface, such as a dielectric, is provided thereby while maintaining superior electrical properties of the capacitor.
Solid electrolytic capacitors with conductive polymers as the cathode are widely used in the electronics industry, and elsewhere, because of their low equivalent series resistance (ESR) and “non-burning” healing mechanism. Typical methods for applying conductive polymers onto a dielectric include in situ chemical/electrochemical oxidation polymerization and coating of preformed conductive polymer dispersions or solutions. Compared with an in-situ polymerization method, coating of preformed conductive polymer is much simpler and less costly.
One particular concern with the conductive polymer coating process is the formation of a defect free polymer coating. As described in U.S. Pat. No. 7,658,986, a continuous coating of all dielectric surfaces by primary cathode materials is essential to prevent short circuit electrical failure. However, as with any coating process, surface tension of the conductive polymer dispersion could cause it to retreat from edges and corners during the drying process resulting in insufficient coverage in these areas. Corners and edges are most susceptible to mechanical or thermal mechanical stress during the capacitor manufacturing process. Without a sufficient polymer layer on the corners and edges of the dielectric in the poorly coated areas may be damaged and subsequent layers, such as carbon and metal layers, can come into direct contact with the dielectric leading to increased residual currents and other reliability issues.
One approach to improving edge and corner coverage is to modify the anode design as disclosed in U.S. Pat. Nos. 7,658,986; D616,388; D599,309 and D586,767. While this approach may be beneficial, it is not a universal method and has limits in practice.
Another approach mentioned by EP-A-1524678, EP-A-1746613 and U.S. Pat. No. 7,411,779 is to mix solid particles with the conductive polymer dispersion. The additional solid particles can be conductive or nonconductive. However, this approach is not always reliable and reproducible. The addition of solid particles often makes the polymeric coating layer brittle, and increases residual leakage and equivalent series resistance (ESR).
Yet another approach is described in WO201089777 and U.S. Pat. No. 8,882,856, which teach the use of a crosslinker solution applied between conductive polymer dispersion dipping cycles to improve polymer coverage of the corners and edges. The effectiveness of the crosslinker is attributed to the presence of multiple cationic functional groups that form a chemical bond, referred to as “crosslinks”, between polymer dispersion particles. While crosslinking does improve coverage on the anode, the crosslinker solution may contaminate the conductive polymer dispersion which causes a viscosity increase of the conductive polymer dispersion. An ion exchange process has been suggested to remove the contamination from the polymer solution or dispersion, however, this adds to manufacturing complexity. Nonetheless, contamination of conductive polymer dispersion by the crosslinker causes more difficulties in process control, and in quality control and requires additional manufacturing steps.
The formed capacitor's initial electrical performance is good with the polyvalent crosslinkers described above. However, U.S. Pat. No. 8,808,403 and U.S. Publ. Pat. Appl. No. 2014/0340819 state that the performance deteriorates over time especially, under humid conditions. The reason is that these crosslinkers are often ionic materials that contain low molecular weight strong ionic species such as sulfate or p-toluenesulfonate. These strong ionic species can dissociate completely in the presence of water and can diffuse through various cathode layers under high humid conditions. The result is higher leakage current or even electrical short failures. This is unacceptable since high humidity is a very common environmental condition for capacitors. The migration of strong ionic species could also cause serious corrosion on certain metals, particularly aluminum. U.S. Pat. No. 8,808,403 suggests the use of a water wash step after the conductive polymer layer is dried to remove the ionic species from the cured conductivity layer. This approach suffers from other tradeoffs such as delamination of polymer from the dielectric, increased ESR and poor ESR stability. An additional washing step also requires a capital investment and is a detriment to manufacturing efficiency.
U.S. Publ. Pat. Appl. No. 2012/0206859 describes four different types of coverage enhancers including amino acids, amine-sulfonic acid salts, quaternary amine halide or sulfonate salts, and nanoparticles. The low molecular weight strong ionic species such as sulfonate and halides, increases the risk of poor leakage performance under humidity.
U.S. Publ. Pat. Appl. No. 2014/0340819 describes the use of diamines, triamines or polyamines in combination with weak acids, such as acids with a dissociation constant or pKa 0.25-6, as a crosslinker. This combination alleviates the corrosion on aluminum anodes, however, as long as a “crosslinker” is involved in the process, the contamination and viscosity increase of the conductive polymer dispersion or solution is still a problem.
Yet another problem with the cross-linkers is that each slurry layer tends to dry and form a skin. Subsequent layers then do not adequately migrate through the skin into previous layers and the layers are joined by adhesion not cohesion. The result is the formation of a cathode wherein discrete layers separate, or delaminate, under harsh conditions thereby decreasing conductivity between adjacent layers and increasing equivalent series resistance (ESR). The stability and reliability of capacitors made with the layered coating of conductive polymer is also undesirable.
U.S. Pat. No. 8,771,381 teaches the application of non-ionic polyol prior to or in between conductive polymer dispersion layers to improve polymer corner and edge coverage. The capacitors are less susceptible to the corrosion caused by ionic species under high humidity conditions, however, the edge and corner coverage enhancement is still inferior.
In spite of the ongoing effort those of skill in the art still do not have a suitable option for the formation of polymer layers from a preformed dispersion which provides adequate coverage of the edges and corners and which is suitable for high humidity conditions. So a need still exists for materials and methods that improve corner and edge coverage of an anode without the negative effect on the leakage performance of the capacitor when exposed to humidity, or on the processability of the conductive polymer dispersion.